This is the process by which a single cell, usually a fertilized egg, becomes a multi-celled organism. It involves the division of cells by mitosis and their differentiation into specialized cells, tissues, and organs.

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Sea urchin embryo cell division

The first 90 minutes of sea urchin (Lytechinus pictus) embryo development. Nuclei are visible as “thumbprints” in each cell. Microtubules are faintly visible just prior to and during mitosis.

Elapsed time about 2 hours.

Sea urchin eggs were fertilized, then immediately mounted in seawater between a slide and coverslip (using a silicon spacer). Images were taken at room temperature on a compound inverted microscope using a 40x DIC objective and digital camera.